Transcript Ch 10 Magma Generation mod 15

You recall discussions of Black Smokers, the placement
of Sulfide deposits at the mid-ocean ridges.
The same thing happens at volcanoes when water and magma are in proximity
For example, at an active Caldera
http://vulcan.wr.usgs.gov/Glossary/Caldera
/description_caldera.html
Petrology Field Trip to Bemco Mining District
Ores from weathered Sulfide deposits
• Mineral deposits
containing sulfide
minerals, e.g. copper
sulfides, are subjected to
weathering, can go into
solution and trickle
down to the reducing
conditions below the
water table, where
native metals or rich
concentrations of ores
are precipitated.
e.g. black smokers, hydrothermal circulations
Gossan Intensely oxidized, weathered or decomposed rock, usually the upper and exposed part of an ore deposit or mineral
vein. In the classic gossan or iron cap all that remains is iron oxides and quartz often in the form of boxworks, quartz lined
cavities retaining the shape of the dissolved ore minerals
.
Solubility in water
The Solubility Rules
1. Salts containing Group I elements are soluble (Li+, Na+, K+, Cs+, Rb+). Exceptions to this rule are rare. Salts containing the ammonium ion (NH 4+) are also
soluble.
2. Salts containing nitrate ion (NO3-) are generally soluble.
3. Salts containing Cl -, Br -, I - are generally soluble. Important exceptions to this rule are halide salts of Ag+, Pb2+, and (Hg2)2+. Thus, AgCl, PbBr2, and
Hg2Cl2 are all insoluble.
4. Most silver salts are insoluble. AgNO3 and Ag(C2H3O2) are common soluble salts of silver; virtually anything else is insoluble.
5. Most sulfate salts are soluble, for example
FeSO4 is soluble. Important exceptions to this rule include BaSO , PbSO ,
4
4
Ag2SO4 and SrSO4 .
6. Most hydroxide salts are only slightly soluble. Hydroxide salts of Group I elements are soluble. Hydroxide salts of Group II elements (Ca, Sr, and Ba)
are slightly soluble. Hydroxide salts of transition metals and Al3+ are insoluble. Thus, Fe(OH)3, Al(OH)3, Co(OH)2 are not soluble.
7. Most sulfides of transition metals are highly
insoluble. Thus, CuS, FeS, FeS2, ZnS, Ag2S are all
insoluble. Arsenic, antimony, bismuth, and lead sulfides
are also insoluble.
8. Carbonates are frequently insoluble. Group II carbonates (Ca, Sr, and Ba) are insoluble. Some other insoluble carbonates include FeCO3 and PbCO3.
9. Chromates are frequently insoluble. Examples: PbCrO4, BaCrO4
10. Phosphates are frequently insoluble. Examples: Ca3(PO4)2, Ag3PO4
11. Fluorides are frequently insoluble. Examples: BaF2, MgF2 PbF2.
Changing insoluble metal sulfides into
soluble sulfates
• Oxidizing Zone above the water table
• Sulfide minerals, for example ferrous
and copper sulfides, are subject to
weathering.
• Sulfide minerals are oxidized near the
surface and produce sulfuric acid. For
example:
• FeS2 (s) + 7O + H2O →FeSO4 (aq) + H2SO4
Formation of the solvent Ferrous Sulfate
The part played by ferric sulfate Fe2(SO4)3 as a solvent can be seen by the following
reactions:
Pyrite
FeS2 + Fe2(SO4)3 → 3FeSO4 + 2S
Chalcopyrite
CuFeS2 + 2Fe2(SO4)3 → CuSO4 + 5FeSO4 + 2S
Chalcocite
Cu2S + Fe2(SO4)3 →CuSO4 + 2FeSO4 + CuS
Covellite
CuS + Fe2(SO4)3 →2FeSO4 + S + CuSO4
Sphalerite
ZnS + 4Fe2(SO4)3 + H2O →ZnSO4 + 8FeSO4 + 4H2SO4
Galena
PbS + Fe2(SO4)3 + H2O + 3O →PbSO4 + 2FeSO4 + H2SO4
Silver
2Ag + Fe2(SO4)3 → Ag2SO4 + 2FeSO4
· Most of the sulfates are readily soluble, and these cold dilute solutions slowly
trickle downwards through the deposit until the proper Eh-pH conditions are met to
cause deposition of their metallic content.
Reaction and Trickling Down
• Iron sulfate reacts with sulfides, they go into solution as
sulfates, acid rainwater then carries, for example copper,
as copper sulfate, down to the water table.
• CuS(s) + Fe2(SO4)3 (aq) →2FeSO4 (aq) + S(s) + CuSO4 (aq)
• The net result is that dissolved copper sulfide
trickles down from the oxidizing upper portion of
the deposit to that portion at and just below the
water table.
Reducing Zone below the water table
• Below the water table, where additional sulfide
minerals remain solid and unoxidized (e.g. Pyrite
FeS2), any iron sulfide grains present will react
with the copper sulfate solution, putting iron into
solution and precipitating copper.
• FeS2 (s) + CuSO4 (aq) → FeSO4 (aq) + Cu(s) + 2S(s)
• This process is called Supergene Enrichment
Hydrothermal Deposit, Bemco Mine
Ch. 10 Origin of
Basaltic Magma
Seismic evidence
in the mantle …
-> basalts are generated
by partial melting of mantle
material
Probably can derive most other magmas
from this primary magma by fractional
crystallization, assimilation, etc.
Basalt is the most common magma
If we are going to understand the origin of
igneous rocks, it’s best to start with the
generation of basalt from the mantle
Chapter 9 has one figure which we should look at for today’s topic:
Here is one corner of that figure:
Basalts in different Plate Tectonic settings are chemically different
Two principal types of basalt
in the ocean basins
Tholeiitic Basalt and Alkaline Basalt
Ocean Islands such as Hawaii have both Tholeiitic AND Alkaline Basalts
Common petrographic differences between tholeiitic and alkaline basalts
Table 10-1
Subalkaline: Tholeiitic-type Basalt
Groundmass
Usually fine-grained, intergranular
Usually fairly coarse, intergranular to ophitic
No olivine
Olivine common
Clinopyroxene = augite (plus possibly pigeonite)
Titaniferous augite (reddish)
Orthopyroxene (hypersthene) common, may rim ol.
Orthopyroxene absent
No alkali feldspar
Interstitial alkali feldspar or feldspathoid may occur
Interstitial glass and/or quartz common
Interstitial glass rare, and quartz absent
Olivine rare, unzoned, and may be partially resorbed
Phenocrysts
Alkaline Basalt
Olivine common and zoned
or show reaction rims of orthopyroxene
Orthopyroxene uncommon
Orthopyroxene absent
Early plagioclase common
Plagioclase later in sequence, uncommon
Clinopyroxene is pale brown augite
Clinopyroxene is titaniferous augite, reddish rims
after Hughes (1982) and McBirney (1993).
A third is hi-Al, or calc-alkaline basalt, usually continental
Subalkaline: Tholeiites and Calc-alkaline Basalts
Example: AFM
diagram
(alkalis-Fe-Mg)
A (Na2O + K2O) , F( FeO + Fe2O3) and M ( MgO )
http://petrology.oxfordjournals.org/cgi/cont
ent/abstract/45/3/507
Figure 8-3. AFM diagram for
Crater Lake volcanics, Oregon
Cascades. Data compiled by Rick
Conrey (personal
communication).
Silica content variation
in two famous Igneous localities
Notice: Skaergard
has a Ferrobasalt
member, Crater Lake
does not.
AFM diagram: Tilley: can further subdivide the subalkaline
magma series into a tholeiitic and a calc-alkaline series
Figure 8-14. AFM diagram showing the distinction
between selected tholeiitic rocks from Iceland, the MidAtlantic Ridge, the Columbia River Basalts, and Hawaii
(solid circles) plus the calc-alkaline rocks of the Cascade
volcanics (open circles). From Irving and Baragar (1971).
After Irvine and Baragar (1971). Can. J. Earth Sci., 8,
523-548.
Above subduction
zones
MORs and Flood
Basalts, and above
Plumes
AFM diagram showing “typical” areas for
various extents of evolution from primitive
magma types. Tholeites go through a
Ferro-Basalt stage before continuing
towards Rhyolite.
Recall Skaergard and
Mt. Mazama
Ophiolite Suite
Some Serpentine is formed
due to hot water
(called Hydrothermal)
circulation
Samples of mantle material

Ophiolites
– Slabs of oceanic crust and upper mantle
– Thrust faulted onto edge of continent
Dredge samples from oceanic fracture
zones
 Nodules and xenoliths in some basalts
 Kimberlite xenoliths Plume passes through a

subduction zone’s carbon
– Diamond-bearing pipes blasted up from the mantle
carrying numerous xenoliths from depth
Lherzolite, Harzburgite and Dunite
Lherzolite is probably fertile unaltered
mantle
 Harzburgite typically forms by the
extraction of partial melts from the more
pyroxene-rich peridotite called lherzolite.
The molten magma extracted from
harzburgite may then erupt on the surface
as basalt. If partial melting of the
harzburgite continues, all of the pyroxene
may be extracted from it to form magma,
leaving behind the pyroxene-poor
peridotite called dunite

Lherzolite is probably fertile unaltered mantle
Dunite and Harzburgite are refractory residuum after basalt has been
extracted by partial melting
Tholeiitic basalt
15
10
Figure 10-1 Brown and Mussett,
A. E. (1993), The Inaccessible
Earth: An Integrated View of Its
Structure and Composition.
Chapman & Hall/Kluwer.
5
Lherzolite
Harzburgite
Dunite
0
0.0
0.2
Residuum
0.4
Wt.% TiO2
0.6
0.8
Lherzolite: A type of peridotite
with Olivine > Opx + Cpx
Olivine
Dunite
90
Peridotites
Lherzolite
40
Pyroxenites
Olivine Websterite
Orthopyroxenite
10
10
Orthopyroxene
Websterite
Clinopyroxenite
Figure 2-2 C After IUGS
Clinopyroxene
Phase diagram for aluminous
Notice the mantle will
4-phase Lherzolite: not melt under normal
Last was Olivive &
Pyroxene, now look at
Al mineralss.
Al-phase =

Ca++ Plagioclase
 shallow

ocean geotherm!
CaAl2Si2O8
.
(< 50 km)
Spinel Lherzolite
Spinel is MgAl2O4
 50-80

km
Garnet Lherzolite
 80-400

km
Si [4] => Si [6]
Mg3Al2(SiO4)3
Si[4]  Si[6] coord.

> 400 km
Figure 10-2 Phase diagram of aluminous Lherzolite with melting interval (gray), sub-solidus
reactions, and geothermal gradient. After Wyllie, P. J. (1981). Geol. Rundsch. 70, 128-153.
How does the mantle melt?
1) Increase the
temperature
Figure 10-3. Melting by
raising the temperature.
No realistic mechanism for the general case because Temps
hotter than the Geothermal Gradient are needed. Maybe
accumulate radioactive decay heat?
Local hot spots OK; very limited area
2) Lower the pressure: MOR and Rifts
– Adiabatic rise of mantle (no conductive heat loss)
– Rise to low pressure, lower MP, “decompression melting”
Steeper than solidus
Intersects solidus
D slope = heat of
fusion as mantle melts
Figure 10-4. Melting by (adiabatic) pressure reduction. Melting begins when the adiabat crosses the
solidus and traverses the shaded melting interval. Dashed lines represent approximate % melting.
Basalt origin 1, at the MOR
3) Add volatiles (especially H2O) lowers Melt Pt
changes slope
Amphibolite
Serpentinite
Eclogite
Eclogite: red
to pink
garnet
(almandinepyrope) in a
green matrix
of sodiumrich pyroxene
(omphacite)
Figure 10-4 or 10-5 (2nd ed). Dry peridotite solidus compared to several experiments on H2O-saturated peridotites.
Basalt origin 2
So, basaltic melts can be created
under several circumstances
We saw: Plates separate and mantle rises at mid-ocean
ridges
 Adiabatic rise  decompression
Melting
 Subduction zones  dewatering

Third way:
Hot spots  melting
plumes, also basaltic
Melting depths vary w\ volcanic province
Most within upper few hundred kilometers
The melts can mix
• There is evidence that plume and MOR
can mix (following slides)
• Certainly a plume rising through a
subduction surface is the favorite model
for diamond transport to the surface.
E-MORBs
• Enriched MORBs (called E-MORBS) have, for
example, higher Lanthanum La, Cerium Ce, and also
higher Strontium Sr than normal N-MORBs
“With increasing depths, the aluminous phase in the upper mantle changes from
plagioclase to spinel to garnet The transition from spinel lherzolite (olivine
+orthopyroxene + clinopyroxene + spinel) to garnet lherzolite (olivine +
orthopyroxene + clinopyroxene+ garnet) could potentially influence the
characteristics of some kinds of basalts, particularly mid-ocean ridge basalts
(MORB), since this transition is thought to occur at about the same depths at
which MORB may originate. There is evidence from trace element and isotope
geo-chemistry that [some, the E-MORBs] MORB are generated in the presence of
garnet (Klein and Langmuir 1987; Hirschmann and Stolper 1996). The evidence
includes the depletion of heavier rare earth elements relative to lighter rare earth
elements (Kay and Gast 1973), depletion in 177Lu/176Hf (Salters and Hart 1989)
and elevated 230Th/238U ratios (Beattie 1993a, b; LaTourrette et al. 1993). This
is generally referred to as the `garnet signature' in MORB. However, if melting
started in the garnet lherzolite stability field, simple melting models (e.g. Klein
and Langmuir 1987; McKenzie and Bickle 1988; Iwamori et al. 1995) predict a
thickness of the oceanic crust much greater than the average crust at 7 +/- 1 km
inferred from seismological studies (e.g. White et al. 1992). Several possible
solutions have been put forward to resolve this apparent conflict, including: (1)
reduced melt productivity of upwelling peridotite (Asimov et al. 1995); (2)
variations in depth of melting on the top of the melting zone beneath ridges
(Shen and Forsyth 1995); or (3) partial melting of small amounts of garnetbearing assemblages in veins such as garnet pyroxenites or eclogites (among
others: Wood 1979; AlleÁgre et al. 1984; Hirschmann and Stolper 1996).”
Klemme and O’Neill (2000)
Lu = Lutetium Hf = Hafnium
Plume and MOR interactions

Origin of enriched-type mid-ocean ridge basalt at ridges far from mantle plumes: The
East Pacific Rise at 11°20′N
Yaoling Niu, Ken D. Collerson, Rodey Batiza, J. Immo Wendt, Marcel Regelous
Journal of Geophysical Research: Solid Earth (1978–2012)
Volume 104, Issue B4, pages 7067–7087, 10 April 1999

The East Pacific Rise (EPR) at 11°20′N erupts an unusually high proportion of enriched mid-ocean
ridge basalts (E-MORBs) and thus is ideal for studying the origin of the enriched heterogeneities
in the EPR mantle far from mantle plumes. These basalts exhibit large compositional variations
(e.g., [La/Sm]N = 0.68–1.47, 87Sr/86Sr = 0.702508–0.702822, and 143Nd/144Nd = 0.513053–
0.513215). The 87Sr/86Sr and 143Nd/144Nd correlate with each other, with ratios of
incompatible elements (e.g., Ba/Zr, La/Sm, and Sm/Yb) and with the abundances and ratios of
major elements (TiO2, Al2O3, FeO, CaO, Na2O, and CaO/Al2O3) after correction for fractionation
effect. These correlations are interpreted to result from melting of a two-component mantle with
the enriched component residing as physically distinct domains in the ambient depleted matrix.
The observation of [Nb/Th]PM > 1 and [Ta/U]PM > 1, plus fractionated Nb/U, Ce/Pb, and Nb/La
ratios, in lavas from the northern EPR region suggests that the enriched domains and depleted
matrix both are constituents of recycled oceanic lithosphere. The recycled crustal/eclogitic
lithologies are the major source of the enriched [E-MORB source] domains, whereas the recycled
mantle/peridotitic residues are the most depleted [N-MORB source] matrix. On Pb-Sr isotope plot,
the 11°20′N data form a trend orthogonal to the main trend defined by the existing EPR data,
indicating that the enriched component has high 87Sr/86Sr and low 206Pb/204Pb and
143Nd/144Nd. This isotopic relationship, together with mantle tomographic studies, suggests that
the source material of 11°20′N lavas may have come from the Hawaiian plume. This “distal
plume-ridge interaction” between the EPR and Hawaii contrasts with the “proximal plume-ridge
interactions” seen along the Mid-Atlantic Ridge. The so-called “garnet signature” in MORB is
interpreted to result from partial melting of the eclogitic [enriched] lithologies. The positive Na8Si8/Fe8 and negative Ca8/Al8-Si8/Fe8 trends defined by EPR lavas result from mantle
compositional (vs. temperature) variation.
Stable Isotopes of Strontium
The ratio 87Sr/86Sr is a parameter often
reported in geologic investigations; ratios in
minerals and rocks have values ranging from
about 0.7 to greater than 4.0. Because
Strontium has an electron configuration similar
to that of calcium, it readily substitutes for Ca
in minerals.
 87Sr/86Sr is used, for example, to distinguish
enriched E-MORBs from depleted source NMORBs.

From a plume
From a plume
Can we generate both tholeiitic
and alkaline basalts from a
chemically uniform mantle?
Variables (other than X)
– Temperature
– Pressure
Figure 10-2 Phase diagram of aluminous lherzolite
with melting interval (gray), sub-solidus reactions, and
geothermal gradient. After Wyllie, P. J. (1981). Geol.
Rundsch. 70, 128-153.
Pressure effects:
Increased pressure
moves the ternary
eutectic minimum from
the oversaturated
tholeiite field to the
under-saturated alkaline
basalt field
Alkaline basalts
are thus favored
by greater depth
of melting
Figure 10-8 Change in the eutectic
(first melt) composition with
increasing pressure from 1 to 3 GPa
projected onto the base of the basalt
tetrahedron. After Kushiro (1968), J.
Geophys. Res., 73, 619-634.
Crystal Fractionation of magmas as they rise


Tholeiite  alkaline
by Fractionation at medium to high Pressure
Recall not at low Pressure, due Albite Thermal
Divide
 Thermal divide, they cannot evolve into one
another, separate sources at low Pressure, but
fractionation at med to high P does allow
evolution of a magma from Tholeiite to Alkaline
Initial Conclusions:
Tholeiites favored by shallower melting
– 25% melting at <30 km  tholeiite
– 25% melting at 60 km  alkaline basalt
 Tholeiites favored by greater % partial
melting
– 20 % melting at 60 km  alkaline basalt

 incompatibles (alkalis)  initial melts
– 30 % melting at 60 km

tholeiite
Initial Conclusions
A chemically homogeneous mantle can
yield a variety of basalt types
 Alkaline basalts are favored over tholeiites
by deeper melting
 Fractionation at moderate to high depths
can also create alkaline basalts from
tholeiites
 At low P there is a thermal divide that
separates the two series

Experiments on melting enriched
vs. depleted mantle samples:
1. Depleted Mantle
Tholeiite easily created
by 10-30% Partial
Melting
 More silica saturated
at lower P

Figure 10-17a. Results of partial melting experiments on
depleted Mantle. Dashed lines are contours representing
percent partial melt produced. Strongly curved lines are
contours of the normative olivine content of the melt. “Opx
out” and “Cpx out” represent the degree of melting at which
these phases are completely consumed in the melt. After
Jaques and Green (1980). Contrib. Mineral. Petrol., 73, 287-310.
Experiments on melting enriched vs.
depleted mantle (DM) samples:
2. Enriched Mantle


Tholeiites extend to
higher P than for
Depleted Mantle
Alkaline basalt field
(purple) at higher P yet
Figure 10-17b. Results of partial melting experiments on fertile
Lherzolites. Dashed lines are contours representing percent
partial melt produced. Strongly curved lines are contours of
the normative olivine content of the melt. “Opx out” and “Cpx
out” represent the degree of melting at which these phases are
completely consumed in the melt. The shaded area represents
the conditions required for the generation of alkaline basaltic
magmas. After Jaques and Green (1980). Contrib. Mineral.
Petrol., 73, 287-310.
At a depth of about 670 – 700 km g Olivine ((Mg,Fe)SiO4 )decomposes into
silicate Perovskite (FeSiO3) and Periclase (MgO) + silica SiO2 in an
endothermic reaction.
Endothermic systems cool, contract, are less buoyant.
This leads some workers to believe that the 670- 700 km boundary blocks
convection from the core mantle boundary, and upper mantle convection
cells are distinct.
Mantle model circa 1975
Figure 10-16a After Basaltic Volcanism Study Project (1981). Lunar and Planetary Institute.
Newer mantle model
Upper depleted mantle = MORB source - Tholeites
 Lower undepleted & enriched OIB source - Alkaline

Boundary = 670
km phase
transition
Sufficient D
density to
impede
convection so
they convect
independently
Figure 10-16b After Basaltic Volcanism Study Project (1981). Lunar and Planetary Institute.